Referring to FIG. 1, a conventional engine 1 includes a cylinder system 11, a piston 12 that is capable of moving upwardly and downwardly in the cylinder system 11, a crankshaft 13 that is driven by the piston 12, a first gear 14 that is disposed on the crankshaft 13, a rotating shaft 15 that is parallel to and space-apart from the crankshaft 13, a second gear 16 that is disposed on the rotating shaft 15 and that meshes with the first gear 14, a water pump 17 that is driven by the rotating shaft 15, and a heat exchanger 18 that is in fluid communication with the water pump 17. When the engine 1 starts to operate, the piston 12 drives the crankshaft 13 along with the first gear 14 to rotate, and the first gear 14 drives the second gear 16 to rotate so as to drive rotation of the rotating shaft 15, thereby activating the water pump 17. The cooling water is transported by the water pump 17 to dissipate the heat generated from the cylinder system 11. The cooling water that has been heated up by the cylinder system 11 is then transported through the water pump 17 into the heat exchanger 18 so as to be cooled down again and reflow into the cylinder system 11 to achieve another cooling cycle.
FIG. 2 is a chart showing a curve of the rotational speed of the crankshaft 13 versus time. FIG. 3 is a chart showing a curve of rotational speed of the rotating shaft 15 versus time. Compare FIG. 2 with FIG. 3, when the rotational speed of the crankshaft 13 changes, the rotational speed of the rotating shaft 15 changes accordingly. However, since the water pump 17 is disposed on a side of the engine 1, the weight distribution is not uniform. Therefore, the rotational speed of the rotating shaft 15 is relatively unstable when the rotating speed of the crankshaft 13 changes, thereby increasing vibration and noise during operation of the engine and reducing the stability and the service life of the engine.